1. Field of the Invention
The present invention relates to a method for forming bumps on the electrodes of semiconductor devices.
2. Prior Art
Conventional methods which use a wire bonding apparatus to form bumps on the electrodes of semiconductor devices are disclosed in, for example, Japanese Patent Application Laid-Open (Kokai) No. 54-2662 and Japanese Patent Application Publication (Kokoku) No. 4-41519.
In the method disclosed in Japanese Patent Application Laid-Open (Kokai) No. 54-2662:
a ball is formed by an electrical discharge, etc., on a wire that passes through the capillary of a wire bonding apparatus, PA1 the capillary is lowered so that the ball is pressed against one of the electrodes of a semiconductor device and bonded thereto, PA1 the capillary is raised slightly, PA1 a clamper is closed so as to clamp the wire, and PA1 the capillary and clamper are both raised so that the ball is separated from the wire and the ball remains on the surface of the electrode as a bump. PA1 the capillary is raised and moved a slight distance in the horizontal direction and is then again lowered, thus forming a weak portion in the wire, PA1 the capillary is raised again, PA1 the clamper is closed so as to clamp the wire, and the capillary and clamper are raised so that the ball is separated from the wire and remains on the surface of the electrode as a bump. PA1 a ball is formed on a wire that passes through a capillary, PA1 the ball is pressed against an electrode on a semiconductor device and bonded thereto, PA1 the capillary is raised by a predetermined amount, and PA1 an ultrasonic vibration is applied to the capillary so as to resonate the wire extending from the lower end of the capillary to the bonded ball, thus causing the wire to be cut at the bonded ball, thereby forming a bump on the electrode. PA1 a ball is formed on a wire which passes through the capillary with the first clamper closed; PA1 the first clamper is opened and the ball is pressed against an electrode on a semiconductor device and bonded thereto, PA1 the capillary is then raised by a predetermined amount, PA1 the second clamper is closed so that the wire is clamped, the timing of the second clamper to be closed being immediately prior to the raising of the capillary, at an intermediate point during the raising motion of the capillary, after the raising of the capillary, or simultaneously with the raising of the capillary, PA1 an ultrasonic vibration is applied to the capillary so that the wire is cut at the bonded ball, thus forming a bump on the electrode, and PA1 the first clamper and capillary are lowered so that the length of wire extending from the lower end of the capillary is adjusted so as to have a tail length which is required for ball formation.
In the method disclosed in Japanese Patent Application Publication (Kokoku) No. 4-41519, the same operation is employed as in the above-identified prior art including the formation of the ball and the pressing and bonding of the ball to the electrode. However, after the ball has been pressed against the electrode and bonded to the electrode,
In the first method described above, a bump is formed merely by raising the capillary and cutting the wire (for separating the ball) after the ball has been pressed against the electrode and bonded. Thus, the length of a minute projection remaining on the upper surface of the bump tends to be long. Furthermore, since the cutting (or separating) position of the ball is not consistent, the length of the minute projection tends to vary greatly.
In the second method described above, an operation in which the capillary is raised, moved horizontally and then again lowered needs to be performed after the ball has been pressed against the electrode and bonded. Thus, this method is inferior in terms of productivity despite the fact that productivity is a very important element in semiconductor manufacturing.